The invention concerns a process for the preparation of photographic color copies of copy masters with a photographic color copy apparatus, whereby the copy master is spectrally measured, in particular in the base colors blue, green and red, and the necessary quantities of copying light determined from the measured data and the copy master copied with the copy light quantities determined onto photographic copy material with known color density curves.
Film materials customarily used in photography, in particular color photography, especially negative films, are characterized by low slopes of the linear segments of the color density curves and a corresponding high exposure latitude. Therefore, with correct exposure, both the shadows and the lights of a high contrast scene may be reproduced with a slight loss of detail contrast on the film material. On the other hand, the usual copy materials available, in particular color papers, display significantly higher slopes of the linear segments of the color density curves and correspondingly have much smaller latitudes than the negative materials. The shadow parts of the copy master are already located mainly in the shoulder of the color density curve while the lights are located in the foot of the color density curve, so that there is a drastic loss of detail contrast. In the case of high contrast negative exposures often copies with false colors and unsatisfactory detail contrasts are obtained.
Particularly high contrast scenes are encountered primarily in snow images, flash exposures and counterlight exposures. In flash exposures, especially flash portraits, facial features are often too "hard." In addition, generally the color tone of the skin color is considered too "cold," which is the result of the spectral compositions of the flash, which is different from the spectral composition (or color temperature) of daylight. Details in shadowy parts are mostly lost. Counterlight exposures, on the other hand usually have more or less extensive sky portions, which in the negative appear to be strongly overexposed compared to other areas important for the overall image. Copies of such masters show no detail in their sky portions. Any attempt to correctly reproduce at least the important part relative to the image of the master in regard to color and density gradation, is usually at the expense of an acceptable reproduction of the brightest portions and the shadows. Further, it is very difficult in manual processing to decide what the parts important relative to the image are and in the case of automatic color copying equipment, is it practically impossible.
In the field of black-white photography, users have available photographic papers with different exposure latitudes. Depending on the contrast range of a copy master or the detail contrast desired, photographic papers with differently sloped linear segments of the blackening curve (gradation) are used. Papers with different gradation may be selected in small series and in amateur operations, while for automatic color copy apparatuses with constantly varying copy masters having different contrast ranges, such a constant change of papers is hardly practical.
DE-C-825 205 describes the problem of preparing soft, harmonic black-white images from the hard or at least high contrast photographic negatives. It is proposed therein to affect the slope of the blackening curve, the gradation of the black-white paper by a metered diffuse exposure of the paper. This exposure is carried out in addition to the principal exposure through the negative. Beginning with a paper of a harder gradation, it is then possible to reduce the gradation, thereby achieving softer black-white copies.
DE-B-1 300 003 discloses that it is possible in the case of monochromatic and color extraction processes and in particular in copying through halftone image fields, to affect the gradation of a photographic material (plate, paper) by a diffuse exposure in addition to the main exposure of the image. This additional exposure may be of the same duration as the principal exposure. The latter is determined by the brightest parts of the image reproduced and the supplemental exposure is determined by the shadow parts.
In DE-C-2 219 849 it is proposed to carry out the photographic reproduction of halftone masters in black-white and in color, and, in particular, the preparation of color extracts (selection negatives), using a normal exposure and a supplemental exposure of the same intensity. No further details of this process are provided by the document cited.
The aforecited documents indicate that it is, in principle, possible in black-white photography in the photographic reproduction of halftone masters and in the preparation of selection negatives (color extracts), to affect the gradation properties of photographic materials (plates, papers) by a supplemental exposure in addition to the normal exposure. However, no indication is given of a process for the preparation of color copies with acceptable coloring and true to detail color copies of high contrast masters.